Method of fabrication of a microwave delay line



July 26. 1966 J. B. BRAUER 3,262,769

METHOD OF FABRICATION OF A MICROWAVE DELAY LINE Original Filed April 24, 1961 United States Patent 3,262,769 METHOD OF FABRICATION OF A MICROWAVE DELAY LINE Joseph B. Brauer, Rome, N.Y., assignor to the United States of America as represented by the Secretary of the Air Force Original application Apr. 24, 1961, Ser. No. 105,241, new Patent No. 3,098,204, dated July 16, 1963. Divided and this application Aug. 7, 1963, Ser. No. 300,710 2 Claims. (Cl. 65-105) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

This application is a division of my application Serial No. 105,241, filed April 24, 1961, now Patent Number 3,098,204, and entitled: Microwave Delay Line and Method of Fabrication.

This invention relates in general to electrical current delay lines, and more particularly to a microwave delay line of piezoelectric material having, as an integral part thereof, input and output transducers.

There are various type of piezoelectric transducers and delay lines known in the prior art. All of such known devices, however, have a limited frequency range due to the precision requirements of their fabrication. For example, a fabrication technique currently employed to eliminate reflections and refractions in a delay line of the type comprehended herein comprises restricting the delay line diameter to one-fourth of the wavelength of the transmitted impulses.

The impractibility of such a technique at microwave frequencies is apparent when one considers the fact that the acoustic wavelength in solids at a frequency of 10,000 megacycles per second (x band) is approximately 5000 angstroms or 0.005 cm. Therefore, assuming that a piezoelectric transducer could be out small enough to provide the fundamental, or lower order harmonics at such a frequenc the precision required would be about twenty-five times closer than that which is usually termed optical tolerance. The composite nature of the delay lines reflected by the present state of the art poses the further problem of excessive losses and spurious signals generated in the bonds between the delay media and the input and output transducers.

A delay line of the type described i the subject of claims in my co-pending application Serial No. 105,241 above identified. The present application concerns more particularly the novel method herein disclosed for fabricating a microwave delay line of the character described. Details of this novel method of my invention will become more apparent from the following descriptions taken in conjunction with the illustrative embodiment in the accompanying drawings, wherein:

FIG. 1 illustrates one method of fabricating a delay line in accordance with the principles of my invention;

FIG. 2 illustrates one embodiment of my invention;

FIG. 3 illustrates another embodiment of my invention; and

FIG. 4 illustrates one embodiment of my invention in combination with input and output means.

Basically, my invention comprehends a microwave delay line having input and output transducers, as an integral part thereof, wherein a single homogeneous piezoelectric crystal is used throughout for both transducers and delay media. It is the use of a crystal media which in itself i capable of serving as a transducer when properly oriented and fed from a microwave cavity that extends the useful frequency range of such a delay line from the ultrasonic range up to the limit Where the wavelength of the propagating signal approaches the unit cell dimensions of the crystal lattice.

The present invention has application chiefly to pulsed radar systems, digital computers, and other military uses where microwave signals are subjected to millisecond time delays. A specific example of such an application is the cross correlating technique used in radar range finding in which signals as received are compared with later received signals by artificially delaying the former after their receipt.

Referring now to FIG. 1 there is illustrated a piezoelectric crystal 11, such as fused quartz, together with heating coil 12. Crystal 11 is cut such that its crystallographic x-axis is perpendicular to faces 19 and 20, and its transducer dimensions are compatible with the frequency to be propagated. The center section of crystal 11 is then heated locally by heating coil 12 until it reaches its working, or melting temperature. Care must be taken to thermally isolate the end portions so that they retain their nature and orientation. The center section of crystal 11 is then drawn into a filament as illustrated by delay media 15 in FIG. 2. The length of said delay media 15 is determined by the desired time delay. Transducers 13 and 14, then, become an integral part of delay media 15 without the necessity of a bond or seal therebetween.

A further refinement of my invention is illustrated in FIG. 3 wherein said delay media has been Wound into a plurality of helical loops 16. Such an arrangement provides greater time delay in a more retricted physical space.

FIG. 4 illustrates one application of the subject delay line. Tuned cavities 17 and 18, together with input waveguide 20, and output waveguide 21, provide input and output means for the subject delay line.

While my invention, by way of illustration, has been thus described with reference to one presently prepared embodiment, it is not intended to be limited thereto. Those killed in the art will readily recognize that many variations of the basic idea will suggest themselves without departing from the spirit or scope of the invention. As an example, it should be apparent that the local heating of the center section of the original crystal may be accomplished by resistance, induction or dielec tric heating, and that the heating element may be shaped to provide exactly the desired heat gradient to control filament size and taper. This and other similar modifications and alterations are deemed to be within the scope of the appended claims.

What is claimed is:

1. The method of fabricating a microwave delay circuit comprising the steps of cutting the ends of a quartz crystal perpendicular to the crystallographic x-axis thereof, thermal isolating the end sections of the crystal, locally heating the center section of said crystal to its working temperature and drawing said center section to provide a filamentary delaying therefrom.

2. A method of fabricating a microwave delay circuit as defined in claim 1 including the further step of form ing said filamentary delay line into a plurality of helical loops.

References Cited by the Examiner 2,485,979 10/1949 McCandless et al. 65102 X DONALL H. SYLVESTER, Primary Examiner. A. D. KELLOGG, Assistant Examiner. 

1. THE METHOD OF FABRICATING A MICROWAVE DELAY CIRCUIT COMPRISING THE STEPS OF CUTTING THE ENDS OF A QUARTZ CRYSTAL PERPENDICULAR TO THE CRYSTALLOGRAPHIC X-AXIS THEREOF, THEREMAL ISOLATING THE END SECTIONS OF THE CRYSTAL, LOCALLY HEATING THE CENTER SECTION OF SAID CRYSTAL TO ITS WORKING TEMPERATURE AND DRAWING SAID CENTER SECTION TO PROVIDE A FILAMENTARY DELAYING THEREFROM. 